A seal ring for sealing an annular gap between a shaft and a housing that rotate relative to each other is provided in an automatic transmission (AT) or a continuously variable transmission (CVT) of an automobile in order to keep the hydraulic pressure. In recent years, amidst an attempt to lower fuel consumption to cope with the environmental challenges, there has been an increasing demand for reducing the rotational torque of such seal rings. Therefore, conventionally, there has been a countermeasure taken to reduce the contact area of the sliding portions between a side surface of an annular groove in which a seal ring is mounted and the seal ring. The seal ring according to this conventional example is now described with reference to FIG. 19.
FIG. 19 is a schematic cross-sectional diagram illustrating the seal ring according to the conventional example in use. The seal ring 300 according to the conventional example is mounted in an annular groove 510 provided on the outer circumference of a shaft 500. The seal ring 300 is making intimate contact with an inner circumferential surface of a shaft hole of a housing 600 through which the shaft 500 is inserted, and is slidably in contact with a side wall surface of the annular groove 510, thereby sealing the annular gap between the shaft 500 and the shaft hole of the housing 600.
The seal ring 300 according to the conventional example is provided with a pair of recessed portions 320 extending in a circumferential direction on inner circumferential side of both side surfaces. For this reason, an effective pressure-receiving region of when the seal ring 300 is pressed by a fluid to be sealed from a high-pressure-side (H) toward a low-pressure-side (L) in an axial direction is a region shown by “A” in FIG. 19. In other words, of the side surface of the seal ring 300, a radial region of a portion 310 where the recessed portion 320 is not formed corresponds to the effective pressure-receiving region A. This is because, within a region provided with the recessed portions 320, the fluid pressure acts from either side in the axial direction thereby offsetting the forces applied to the seal ring 300 in the axial direction. Note that the area over the entire circumference of the pressure-receiving region A is the effective pressure-receiving area with respect to the axial direction.
Further, an effective pressure-receiving region of when the seal ring 300 is pressed radially outward by the fluid to be sealed from an inner circumferential surface side toward an outer circumferential surface side is a region shown by “B” in FIG. 19. In other words, the thickness of the seal ring 300 in the axial direction corresponds to the effective pressure-receiving region B. Note that the area over the entire circumference of the pressure-receiving region B is the pressure-receiving area with respect to the radial direction.
As described above, by setting the relationship [length of region A]<[length of region B], it becomes possible to cause the seal ring 300 and the side wall surface of the annular groove 510 to slide against each other. In addition, the rotational torque can be reduced by making the length of the pressure-receiving region A as short as possible.
However, the contact region of the seal ring 300 against the side wall surface of the annular groove 510 is a region shown by “C” in FIG. 19. In other words, of a side surface of the seal ring 300 on the low-pressure-side (L), and of the portion 310 where the recessed portion 320 is not formed, only the part excepting the part exposed to the gap between the shaft 500 and the housing 600 comes into contact with the side wall surface of the annular groove 510. Accordingly, the contact region C of the seal ring 300 is affected by the size of the gap between the shaft 500 and the housing 600. Therefore, depending on the usage environment, the area of contact between the side wall surface of the annular groove 510 and the seal ring 300 may become excessively small, resulting in a degradation of the sealing performance of the seal ring 300. In addition, depending on the usage environment, the contact region may vary continually, resulting in an unstable sealing performance.